Natasha trusted them. Really, she did. Everyone always assumes that she is a cold hearted killer with no capacity for love. That was true. But it didn't mean she couldn't trust them. She just didn't express her trust in the most obvious ways. She showed it in little emotions and gestures. Leaving her back unguarded, roaming the tower without her weapons. Nobody noticed, but these little actions were there. She trusted Cap because as long as you were fighting on his side he would save you in a heartbeat. She remembered how he risked her life in Germany for one, old stranger. And Natasha knew, if that had been her, he would have stood up for her to.
And Natasha definitely trusted Clint. She would have to be crazy not to. He was her partner on every mission. He had saved her life and turned her S.H.I.E.L.D. He ignored commands just to save her. And she was so very grateful. Natasha kept up with the news. She knew if he had ignored her, she would have been dead the next day. And Clint has saved her life more than just that time. She definitely trusted Clint.
Bruce Banner. The Hulk. They all thought that she was uncomfortable with the thought of a wildcard. And she was. Natasha definitely did not like the idea that Bruce could suddenly turn into a monster. But she trusted Bruce. He was smart, and gave up a comfortable life to go help people in third world countries. He wouldn't hurt a fly. More importantly, Natasha trusted the Hulk. She knew that Hulk would fight to protect his friends. No, Natasha simply didn't like that Bruce could transform against his will. That unsettled her. But Natasha trusted her teammate and that is what counts.
Thor was an alien. Just a fact, not an opinion. He had powers over thunder and lightning. He was overly trusting. And he wasn't the smartest of the bunch. Those are the facts. But Natasha knew there was more to him than that. Thor was a prince. He was not overly trusting, and he was not as dumb as he seemed. He was a prince. He had to protect himself from the thousands who wanted to do him harm. Natasha respected that. Natasha understood that. Natasha also knew that when the time was right, when Thor trusted them, he would reveal it. So Natasha would wait.
Natasha trusted Tony Stark. She knew that she shouldn't. He was the real wildcard. He was unpredictable, unstable, and unreliable. He was driven solely by emotions and had no pattern to his action. But he was driven solely by emotions and as long as he thought you were doing the right thing, he would fight for you until his dying breath. Natasha knew she shouldn't trust Tony Stark, but she wanted to. He fought for the good, and it had been so long since Natasha did that as well. So Natasha would always trust Tony Stark.